Techniques are known for forming opto-electronic devices such as light-emitting diodes (LEDs) over a substrate. Supported on the substrate, each such device comprises a layer of electroluminescent material disposed between two electrodes referred to as the anode and cathode. The device may also comprise a charge injection layer and/or a charge transport layer disposed between the electroluminescent layer and one of the electrodes. In modern devices, the electroluminescent layer, charge injection layer and/or charge transport layer may be formed of an organic polymer. An LED comprising one or more such organic layers may be referred to as an organic LED (OLED). In a particular application, an array of these constituent devices may be formed over a substrate such as glass or plastic in order to produce a unit such as an electronic display screen for a computer, television, mobile terminal or other appliance.
FIG. 1 shows a schematic top-down view of an exemplary display panel. The display panel comprises a substrate 102 having an array area 104 in which the pixels of the display are formed. Within the array area 104 are defined a plurality of structural lines in the form of channel regions 106. Each line or channel region 106 comprises a row of smaller pixel regions 108 formed at regular intervals along the length of the channel region 106. The substrate 102 and array area 104 are typically both square or rectangular shaped, in the same orientation, hence defining perpendicular axes X and Y in the horizontal and vertical directions of the array area as shown, with the mutually perpendicular Z axis being perpendicular to the plane of the substrate 102 and array area 104 (i.e. out of the page). In this case the pixel regions 108 are typically arranged in rows along the horizontal length of the array area (the X direction) and columns along its vertical length (the Y direction). In the example shown the channel regions 106 have a longitudinal direction which extends across the horizontal length of the array area 104 (in the X direction). The pixel regions 108 themselves each also have a longitudinal axis aligned with their respective channel 106, each being longer in the direction along the channel 106 than in the direction perpendicular to the channel 106.
FIG. 2 shows a schematic side cross-section through the layers of an exemplary light-emitting device such as an OLED, which may be formed in a pixel region 108 of a display panel. The method of forming the panel begins by providing a substrate 202 and then forming portions of anode material 204 over the substrate 202. Preferably the substrate 202 is formed of a transparent material such as glass or a suitable plastic, and the anode 204 is formed of a transparent conductor such as indium tin oxide (ITO), such that the light can be emitted through the substrate 202.
Each channel region 106 may be defined by a longitudinal well formed over the substrate 102 in the array area 104 (running in the X direction in the example of FIG. 1). In this case, each well is formed by banks of a separating material 214 such as photoresist defining the sides of the well, and a strip of the anode material 204 at the base of the well. Each channel region 106 has a structurally recurring feature along its length which defines the separate component pixel regions 108, e.g. a repeated tapering and/or break in the well at regular points along its length (i.e. at periodic X coordinates in the example of FIG. 1). In the pixel regions 108 of the well, a hole injection layer 206 is then formed over the anode 204, a hole transport layer 208 is formed over the hole injection layer, a semiconducting electroluminescent layer 210 is formed over the hole transport layer 208, and a cathode 212 is formed over the electroluminescent layer 210.
In operation of the finished device, when a suitable potential difference is applied between the anode 204 and cathode 212, holes (h+) are injected into the device from the anode 104 and electrons (e−) are injected from the cathode 212. The holes and electrons combine in the electroluminescent layer 210 to cause an excitation which then decays to emit light. Injection of the holes from the anode 204 is assisted by the hole injection layer 206, and transport of the holes from the anode 204 to the electroluminescent layer 210 is assisted by the hole transport layer 208.
Alternative arrangements are also possible, for example with a cathode formed over the substrate and the anode as the top electrode, and/or comprising an electron injection layer and electron transport layer between the cathode and an electroluminescent layer.
As mentioned, each of the hole injection layer 206, hole transport layer 208 and/or electroluminescent layer 210 may be formed of an organic polymer. An advantage of organic polymer layers is that they can be deposited by solution-processing of a film-forming polymer material, allowing low cost manufacturing and better control over the deposition process. The technique involves applying a solution containing the desired polymer material to the relevant surface, then drying off the solvent by evaporation or other such drying technique, thereby leaving a film of the remaining polymer material. Advantages of organic polymers along with some suitable solutions and deposition techniques are discussed in international patent application publication no. WO 2006/123167.
For forming the hole injection layer 206, the solute used may be a hole-injecting material comprising an “active” component doped with a typically larger amount of host “matrix” component. The active component is that actually chosen for its ability to promote hole injection, whilst the matrix component is a charge balancing counterion. One particular example of a hole-injecting material disclosed in WO 2006/123167 is PEDOT: PSS. That is, an active component of polyethylene-dioxythiophene (“PEDOT” or sometimes just “PEDT”) doped with a matrix component of polystyrene sulfonate (“PSS”). The PSS is soluble in water so as to produce a solution suitable for deposition techniques. Recently there have been provided alternative hole-injection materials other than PEDOT:PSS. For example, Plextronics produce a range of materials referred to as “PLEX” comprising a sulfonated polythiophene derivative as an active component and poly-(4-vinylphenol) (PVPh) as a matrix component. Unlike PEDOT:PSS, an additional solvent such as diethylene glycol is used in the deposition solution.
One technique that can be used for depositing features of the panel such as the organic layer(s) 206, 208, 210 is inkjet printing, e.g. using inks comprising solutions of organic conducting or semiconducting polymers to deposit the organic layers. Note that ink in this context does not necessarily imply colouring, but rather can refer to any solution that can be printed and then dried in order to deposit the solute.
To this end, a print-head 110 may be provided comprising a row of nozzles 112. The row of nozzles is oriented parallel with the direction of the channel regions 106 (the X direction in FIG. 1), and the print-head 110 and substrate 102 move relative to one another so as to perform a printing pass in a print direction P transverse to the channel regions 106 (i.e. moving in the Y direction in FIG. 1). Moving relative to the substrate 102 in this manner, the print-head 110 thus prints into each channel region 106 in succession. Further, the nozzles 112 are spaced with a regular spacing along the row, preferably with multiple nozzles per pixel region 108. Thus as the print-head 110 moves over each successive channel 106 in turn, it prints multiple drops of the organic ink into each pixel region 108 of the respective channel 106 simultaneously.